As a structure for a fuel filling section for filling fuel into the fuel tank of an automobile or the like, a structure having a valve body (opening/closing valve) that is usually closed and that opens against the urging force of a spring when pushed by a fuel filling gun, is disclosed in Patent Document 1. When the pressure within a tank main body becomes a negative pressure of a predetermined value or more, the valve body opens slightly against the urging force of the spring such that the pressure within the tank main body does not become greater than or equal to a predetermined pressure. Further, the structure of Patent Document 1 is provided with a positive pressure valve that opens when the pressure within the tank main body becomes higher than a valve opening pressure that is set by the spring force.
However, in the structure of Patent Document 1 and the like, the urging force of the spring that urges the valve body (the opening/closing valve) is set to be high in order to improve the sealability by the valve body and in order to suppress rattling, and therefore, an urging force, i.e., a valve opening pressure, that corresponds to the magnitude of the negative pressure that arises within the fuel tank main body, cannot be set.    Patent Document 1: Japanese Patent Application Laid-Open No. 8-142694